Multicolor Photochromism of Fulgide Mixed Crystals with Enhanced

Article pubs.acs.org/crystal

Multicolor Photochromism of Fulgide Mixed Crystals with Enhanced Fatigue Resistance Roshan K. Weerasekara,†,‡ Hidehiro Uekusa,§ and Champika V. Hettiarachchi*,†,‡ †

Postgraduate Institute of Science and ‡Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka § Department of Chemistry and Materials Science, Tokyo Institute of Technology, Tokyo 152-8550, Japan S Supporting Information *

ABSTRACT: Two photochromic mixed fulgide single crystal systems showing multicolor photochromism and enhanced fatigue resistance were prepared with isostructural molecular pairs containing methyl-chloro and methyl-bromo groups. Parent crystals p-methylacetophenylisopropylidenesuccinic anhydride (1E), p-chloroacetophenylisopropylidenesuccinic anhydride (2E), and p-bromoacetophenylisopropylidenesuccinic anhydride (3E) showed light magenta, brownish orange, and orange colors, respectively, upon irradiation at 365 nm. Each two component mixed crystal MIX-1E and MIX-2E composed of 1E/2E and 1E/3E in a 0.39:0.61 and 0.88:0.12 ratio, showed four different colors upon irradiation with UV light and visible light of appropriate wavelengths. Further, it was found that colors of mixed crystals can be fine-tuned to impart different shades of colors over a range of four distinct colors shown by each, upon changing the length of the time irradiated at selected wavelengths. Such multicolored photochromic systems can be used to develop optoelectronic devices upon further improvements.

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INTRODUCTION A reversible transformation between two isomers having different absorption spectra, chemical and physical properties, by photoirradiation at two different wavelengths is called photochromism.1,2 These changes make photochromes much more useful in various applications especially in the field of optical electronics3−6 and in photochromic lenses.7 Molecular level electronic systems must have various advantages such as high resolution, high speed of writing and erasing, multiplex recording capability, etc. over the conventional heat-mode recording systems.3 Among organic photochromic systems, fulgide (bismethylenesuccinic anhydride with at least one aromatic ring on the exo-methylene carbon atoms) is a major compound exhibiting photochromism in solutions and in the solid state due to the formation of the closed C-isomer with a chromophore having extended conjugation (Scheme 1). Simultaneously, the E-isomer can isomerize to the Z-isomer as well on UV irradiation.1,8 Even though many photochromic fulgides have been synthesized by many research groups, still it is difficult to obtain their photochromism in solid matrices or in the crystalline state with fatigue resistance and thermal stability as high as in solutions.1,9−11 Thus, their photochromic properties in the crystalline state as well as in condensed matrices have to be improved further, to use them in fabrication of optical electronic devices with high efficiency, fatigue resistance, and thermal stability.12 Meanwhile, it was reported that multicomponent systems consisting of two or three different diarylethane molecules © 2017 American Chemical Society

showing different photochromic colors are of great interest, since they can exhibit multicolor photochromism and other properties in the single crystal form, and possess high durability, high efficiency of photocoloration, and molecular-scale high resolution. Such systems have the possibility of usage in fabricating three-dimensional (3D) memory devices and multicolor displays over the physically mixed composites.13,14 Even though a mixed fulgide composite had been prepared by blending two fulgides in the Poly(methyl methacrylate) (PMMA) polymer matrix exhibiting two colors,15 still no fulgide multicolor photochromic systems in the single crystal form have been formulated. On the other hand, successful formation of substitutional solid solutions between isostructural molecular pairs containing methyl, chloro, and bromo groups were reported several times.16−20 Hence, it would be of a great interest if a mixed crystal is formed between fulgides by exchanging methyl, chloro, and bromo groups, which can show multicolor photochromism. Thus, three new fulgides;(E)-p-methylacetophenylisopropylidenesuccinic anhydride (1E), (E)-p-chloroacetophenylisopropylidenesuccinic anhydride (2E), and (E)-p-bromoacetophenyl isopropylidenesuccinic anhydride (3E) comprising of methyl, chloro, and bromo groups, respectively, at the para positions of the benzene rings of the aromatic alkylidene moiety were synthesized. As expected, they showed light magenta, brownish Received: November 23, 2016 Revised: April 6, 2017 Published: April 13, 2017 3040

DOI: 10.1021/acs.cgd.6b01708 Cryst. Growth Des. 2017, 17, 3040−3047

Crystal Growth & Design

Article

Scheme 1. Photochromic Reaction of a Fulgide

Scheme 2. Structures and Abbreviations of Synthesized Parent Fulgides

(0.0565 mol, 6.70 g) in tert-butanol (60.00 mL) over 20 min and refluxed further for 30 min. The above crude product (10.0 mL) was esterified with 10% conc. HCl (10.00 mL) in absolute ethanol (100.00 mL). Then, in the second Stobbe condensation, the mixture of the above product (9.45 g) and p-methylacetophenone (0.05 mol, 8.00 mL) in toluene (10.00 mL) was added to a refluxing solution of NaH (60.00% dispersion in mineral oil, 0.125 mol, 5.00 g) in toluene and refluxed further for 2−3 h. The crude product (7.62 g) was hydrolyzed with 10% KOH (10.00 g) in absolute ethanol (100.0 mL) by refluxing for 6−8 h. Finally the hydrolyzed crude product (5.78 g) was cyclized with acetic anhydride (150.00 mL), and resultant E and Z isomers of fulgide were separated from the byproducts using a gravity column (silica gel 230−400 mesh) using 2% ethyl acetate in hexane as the mobile phase. Crystals suitable for single crystal X-ray data collection were grown from acetone at ambient temperature. mp 156−158 °C, IR data: υmax 1766.79 cm−1, 1801.51 cm−1 (CO anhydride). Percentage yield = 6%, 0.81 g. Synthesis of (E)-p-Chloroacetophenylisopropylidenesuccinic Anhydride (2E). Procedure for the synthesis of 1E was repeated to synthesize 2E as well, using p-chloroacetophenone (0.05 mol, 7.00 mL) instead of p-methylacetophenone. Final product was recrystallized from acetone at ambient temperature to obtain good quality crystals suitable for single crystal X-ray diffraction data collection. mp 136−138 °C, IR data: υmax 1764.86 cm−1, 1809.22 cm−1 (CO anhydride). Percentage yield = 3%, 0.39 g. Synthesis of (E)-p-Bromoacetophenylisopropylidenesuccinic Anhydride (3E). 3E was synthesized following the same procedure used for the synthesis of 1E, using p-bromoacetophenone(0.05 mol, 10.15 g) in place of p-methylacetophenone. X-ray diffraction quality crystals were grown from acetone. mp 138−140 °C, IR data: υmax 1770.65 cm−1, 1813.08 cm−1 (CO anhydride). Percentage yield = 4%, 0.56 g. Formation of Mixed Crystals. Two mixed crystals MIX-1E and MIX-2E were obtained by cocrystallizing 1:1 molar mixtures of parent fulgide pairs, 1E and 2E and 1E and 3E, respectively, from acetone at ambient temperature, by slow evaporation of the solvent. Melting points of MIX-1E and MIX-2E were 134−136 °C and 132−134 °C, respectively.

orange, and orange colors, respectively, in their closed C-isomer forms, upon irradiation with a UV light at 365 nm. Consequently, three mixed crystal systems were formed by mixing chloro-methyl, bromo-methyl, and chloro-bromo fulgide pairs, and only two mixed crystals comprising methyl-chloro (MIX-1E) and methyl-bromo (MIX-2E) combinations revealed four colors in their single crystal forms, while mixed crystals formed a mixing chloro-bromo combination did not show four color states distinguishably upon irradiation. Herein we report the synthesis of 1E, 2E, 3E, MIX-1E, and MIX-2E, their crystal structures, and photochromic properties in the crystalline state.

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MATERIALS AND METHODS

All solvents were purified by standard techniques prior to use, and Sigma-Aldrich products of potassium tert-butoxide, tert-butanol, pmethylacetophenone, p-chloroacetophenone, p-bromoacetophenone, acetic anhydride, and potassium hydroxide were used as received without further purification. All glassware were oven-dried overnight prior to use. Melting points were recorded on a Stuart Scientific capillary melting apparatus and reported without correction. Fourier transform infrared (FTIR) spectra of pellets prepared containing 3% sample in KBr were measured on a SHIMADZU IR-PRESTIGE-21 spectrophotometer. Solid state UV−visible spectra were recorded using a 1% sample in BaSO4 pellet on a SHIMADZU UV-2450 UV− visible spectrophotometer. UV light and white light irradiations were carried out at 365 nm with a Vilber Lourmat UV lamp (50/60 Hz, 6W UV bulb) and a white light of 6 W halogen lamp (>450 nm) respectively. Irradiation was carried out by keeping the distance between the sample pellet and the irradiation lamp at 1 cm. Monochromatic radiation was obtained from an AMKO grating monochromator (AMKO SN 96006) with a 150 W Xe collimated arc lamp light source. Synthesis of (E)-p-Methylacetophenylisopropylidenesuccinic Anhydride (1E). In first Stobbe condensation, a mixture of diethylsuccinate (0.0625 mol, 10.60 mL) and acetone (0.05 mol, 3.00 mL) was added to a refluxing solution of potassium tert-butoxide 3041



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Table 1. Crystallographic Data of Crystals 1E, 2E, 3E, MIX-1E, and MIX-2E formula formula weight/g mol−1 crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg v/Å3 Z F(000) reflections collected independent reflections no. of restraints/parameters goodness-of-fit on F2 R1 (I > 2σ(I)) wR2 (all data) largest diff peak/hole [e Å−3] mixed crystal molecular ratio

1E

2E

3E

MIX-1E

MIX-2E

C16H16O3 256.29 monoclinic P21 8.8772(6) 7.5252 (5) 10.5908 (8) 90.00 104.087 (2) 90.00 686.22 (8) 2 272 6673 3003 1/180 1.108 0.040 0.127 0.14, −0.17

C15H13ClO3 276.71 monoclinic P21/c 12.9138(9) 6.9356(6) 14.7602(12) 90.00 94.845(2) 90.00 1317.27(18) 4 576 12298 2960 0/175 1.059 0.051 0.151 0.35, −0.38

C15H13BrO3 321.16 monoclinic P21/c 13.0032(9) 6.9610(4) 15.1201(1) 90.00 94.898(4) 90.00 1363.60(17) 4 648 14875 2484 0/175 1.096 0.069 0.210 1.04, −0.73

C15.39H14.18Cl0.61O3 268.66 monoclinic P21 8.7710(3) 7.4059(2) 10.5578(3) 90.00 103.699(1) 90.00 666.30(3) 2 282 7555 2336 13/186 1.046 0.035 0.098 0.17, −0.14 0.39:0.61

C15.88H15.64Br0.12O3 264.07 monoclinic P21 8.8120(8) 7.4460(7) 10.5023(11) 90.00 104.246(3) 90.00 667.91(11) 2 278 6489 3007 9/186 1.122 0.047 0.112 0.19, −0.18 0.88:0.12

X-ray Crystallographic Analysis of Parent and Mixed Fulgide Crystals. Three-dimensional intensity data for the three parent crystals 1E, 2E, and 3E and two mixed crystals MIX-1E and MIX-2E were collected on a RIGAKU RAXIS RAPID diffractometer quipped with an imaging plate camera and with Cu or Mo Kα radiation obtained from a fine focused sealed tube generator equipped with a graphite-monochromator. Except for 1E, for all other crystals, data were collected at 173 K. The integrated and scaled data were empirically corrected for absorption effects using ABSCOR.21 All structures were initially solved by direct methods using the program SIR-92,22 and structures were completed and refined by full-matrix least-squares method on F2 along with SHELXL97.23 In three parent crystals, all non-H atoms were refined anisotropically and H atoms were kept in calculated positions. During the isotropic refinement of H atoms, their coordinates were refined with Riding model, and C−H distances of some H atoms were also refined along with Riding model by keeping isotropic temperature factors at a constant value. In the difference Fourier map of MIX-1E, a peak with very high intensity was found about 1.74 Å away from C13 atom in the benzene ring and that was assigned to the chloride atom of the 2E and refined isotropically. After the refinement, another peak with significantly high intensity was observed about 1.52 Å away from C13 and that was assigned as the C16 carbon atom of the methyl group of 4methylacetophenyl moiety in 1E. Hydrogen atoms of C16 were introduced geometrically and refined isotropically with the Riding model. The two disordered groups Cl and C16 were refined with PART command by constraining the sum of the site occupancy factors of them to be at 1.00 in the refinement in order to obtain the information on ratio between chloro and methyl containing fulgides in the mixed crystal formed. In the final refinement cycles, all non-H atoms were refined anisotropically, but C16 isotropically and with that R-factor was improved to have a value less than 5% indicating the close similarity between the actual crystal structure and the modal structure deduced. In the refinement of MIX-2E, during the structure completion part, there was a high intense peak ∼1.55 Å away from the C13 atom in the benzene ring, and it was assigned to the methyl group carbon atom C16. During the refinement cycles with anisotropic temperature factors for all non-H atoms, there was another peak with a considerably high intensity about 1.81 Å away from C13 and that was named as the bromine atom of the 4-bromoacetophenyl group of 3E component in the MIX-2E and refined anisotropically. Hydrogen atoms of the C16 were introduced geometrically and refined with the

Riding model. Like in MIX-1E, PART command was used, and the sum of the site occupancy factors of C16 and Br was constrained to be 1.00 in the refinement to obtain the ratio between bromo and methyl containing fulgides in MIX-2E crystallographycally. Photochromism in Parent Fulgide Crystals and Multicolor Photochromism in Mixed Crystals. Photocoloration of 1E, 2E, 3E, MIX-1E, and MIX-2E was observed by UV irradiating crystals, while photobleaching was observed in all colored crystals on irradiating them with white light. In order to investigate the multicolor photochromism in the mixed crystal form, crystals of MIX-1E and MIX-2E were also irradiated, first with 365 nm UV light followed by irradiating with the monochromatic visible light corresponding to the λmax value of the visible peak of the photogenerated C-form of each parent fulgide, 1E, 2E, and 3E. The colored crystal images were taken with a SONY 14.1 Mega Pixels digital camera coupled with a light microscope. Determination of Fatigue Resistance of Parent and Mixed Fulgides in the Crystalline State. Fatigue resistance of three parent fulgides and two mixed fulgide systems in the crystalline state was determined using UV−visible spectroscopic data. Each sample disk was prepared as 1% of fulgide in BaSO4. Initially, the baseline was corrected using two BaSO4 disks. One BaSO4 disk was replaced with the sample disk, and an initial UV−visible spectrum was recorded. Then, it was irradiated for 5 min with the UV light (365 nm), and the UV−visible spectrum of the colored pellet was recorded. The colored pellet was then bleached by irradiating with the white light (>450 nm) for about 6 min or more until the absorbance of the visible peak was zero and the UV−visible spectrum of the colorless pellet was recorded. Again, the bleached sample pellet was irradiated with the UV light for 5 min, and the spectrum of the colored pellet was recorded. Then, the colored sample pellet was irradiated again with the white light for about 6 min, and the UV−visible spectrum of the colorless pellet was recorded again. This procedure was repeated for a number of coloring and bleaching cycles until the absorption of the visible peak was closer to zero after UV irradiation.

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RESULTS AND DISCUSSION Molecular Structures of Fulgides in Parent and Mixed Crystals. Crystal morphologies of the two mixed crystal systems, obtained by cocrystallizing compound pairs, were found to be different from each of their parental crystal 3042



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Figure 1. Molecular structures of fulgides in parent and mixed crystals. Ellipsoids were drawn at 50% probability.

With respect to Figure 1, molecular structures of 1E, 2E, and 3E are almost similar having differences only in the pendent groups at the para positions. Since molecular pairs used to make MIX-1E and MIX-2E are isostructural, the structural disorder was found only at the para positions of the benzene rings, while other parts of each molecular pairs are superimposable in both mixed crystals, as anticipated. Generally, continuous mixed crystals or substitutional molecular solid solutions can be formed in different compositions only between isostructural and isomorphous molecular pairs by replacing one kind of molecule by the other type especially between the molecular pairs consisting of chloro-methyl or bromo-methyl or chloro-bromo groups as pendent groups with the same positions, because of comparably similar sizes in three groups.16−20 As opposed to the occupancy ratios obtained from crystallographic data, an HPLC run for parent crystals and mixed crystals showed that ratios between two components available in each mixed crystal are not similar to the ratios obtained crystallographically (Figure 1S). Therefore, there is a possibility of forming thermodynamically stable series of mixed crystals possessing any ratio between the two components mixed, exhibiting a continuous mixing ability between each pair used.

morphologies. Crystallographic data collected for crystals of 1E, 2E, 3E, MIX-1E, and MIX-2E are given in Table 1. In accord with Table 1, 2E and 3E fulgide crystals, containing halogens, belong to a centrosymmetric space group, P21/c, while the space group of the crystals of fulgide 1E with 4methyl group belongs to an on-centrosymmetric, P21 in their pure crystal forms. Hence, each pair mixed belongs to different space groups adopting different crystal structures showing nonisomorphism between them. As revealed by site occupancy factors of Me, Cl, and Br groups in mixed crystals, according to crystal structure analysis data, Me/Cl and Me/Br ratios are 0.39:0.61 and 0.88:0.12 for MIX-1E and MIX-2E respectively, for each representative mixed crystal chosen for data collection. Therefore, in spite of the major component 2E present in the MIX-1E, the minor component 1E (39%) must have become the host compound here, because the unit cell parameters and space group of MIX-1E are similar to those of 1E. This implies that in MIX-1E formation, 61% of 1E host molecules have been replaced by 2E guest molecules. This is a very strange and rare situation, because generally, in mixed crystals, cell parameters and the space group of the mixed one are similar to those of the major component.18,20,24 Contrastingly, an equimolar mixture of 1E and 3E cocrystallized to give MIX-2E having cell parameters closer to those of the host component 1E, and here only 12% of 3E has been incorporated as the guest component. 3043



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Figure 2. Light microscopic images of crystals 1E, 2E, and 3E before and after UV irradiation (magnification 40×).

Photochromism of Parent Fulgides in Crystalline State. Three isostructural parent fulgides 1E, 2E, and 3E underwent photochromic reactions in both solutions and single crystal form reversibly, showing light magenta, brownish orange, and orange colors, respectively, upon irradiation with UV light (365 nm), as revealed in Figure 2. This coloration is attributable to the formation of photogenerated C-forms (closed forms) 1C, 2C, and 3C respectively, and they were converted back to E-forms completely on irradiating for a few minutes with the visible light (λ> 450 nm), by bleaching the crystals completely. According to solid state UV−visible spectra plotted in Figure 3, the absorption maxima (λmax) of the photogenerated C-forms are at 517, 547, and 541 nm for the fulgides of 1E, 2E, and 3E, respectively. Multicolor Photochromism in Two Component Mixed Fulgide Crystals. Multicolor photochromic properties of two selected mixed crystals were investigated carefully to observe the possibility of exploiting them in photoswitchable multicolor displays. A light yellow crystal of MIX-1E turned to reddish pink as shown in Figure 4, upon irradiation with the UV light (365 nm) for 5−7 min, and it can be attributed as the merged colors of two photogenerated C-forms of component compounds, 1E (light magenta) and 2E (brownish orange). Thus, a broad visible peak maximizing at 536 nm appeared in the spectrum of MIX-1E, which is in between the λmax values of the two parental C-forms. Upon irradiation with the white light (λ > 450 nm), the reddish pink MIX-1E crystal bleached completely as a result of the formation of open E-isomers again in component fulgides. Furthermore, when the reddish pink crystal was reirradiated for several minutes at 547 nm with monochromatic radiation, it partially bleached to orange pink as depicted in Figure 4c. Since 547 nm is the absorption wavelength of the C-isomer of the 2E fulgide, most of the brownish orange C-isomers formed by 2E in MIX-1E converted back to open, light yellow E-isomers again. Therefore, the orange pink color observed in the MIX1E crystal must be due to the presence of the photogenerated C-form of 1E and remaining unreacted C-forms of 2E. On the other hand, when MIX-1E irradiated with 365 nm UV light was reirradiated for several minutes with the monochromatic green light (∼517 nm), the crystal turned to brownish orange as illustrated in Figure 4d by imparting the color of the nonreacted C-form of 2E. Therefore, on blue light irradiation, only 2C-isomers of 1E selectively bleached back to the open 1E-isomers by indicating the partial reactivity in mixed crystal form. Similarly, when a light yellow MIX-2E crystal was irradiated at 365 nm, the crystal color changed to brownish red (Figure 4f) due to the presence of photogenerated C-forms of both 1E and 3E fulgides together. Interestingly, upon

Figure 3. Solid state UV−visible absorption spectra of (a) 1E, 2E, and MIX-1E after UV irradiation (365 nm), (b) 1E, 3E, and MIX-2E after UV irradiation (365 nm).

irradiation at ∼541 nm for several minutes, the brownish red MIX-2E crystal changed to light magenta, which is the color of the C-isomer of 1E. During the monochromatic 541 nm light irradiation, only the closed C-isomers of 3E have returned to the light yellow E-isomers, while leaving the C-isomers of 1E in unreactive form. On the other hand, the brownish red crystal became light orange, after reirradiation with the green light (∼517 nm) for several minutes. The reason is C-isomers of the 1E having light magenta color bleached back to E-isomers by imparting the color of the C-form of 3E into the MIX-2E. Same as in MIX-1E, the brownish red MIX-2E crystal bleached completely upon irradiation with white light resulting in the conversion of all C-isomers of both 1E and 3E to their open forms. 3044



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Figure 4. Light microscopic images of MIX-1E (a) before UV irradiation (b) after irradiation with 365 nm UV light (c) irradiated with UV light followed by 547 nm light (d) irradiated with UV light followed by 517 nm light (magnification 40×). Light microscopic images of MIX-2E (e) before UV irradiation (f) after irradiation with 365 nm light (g) irradiated with UV light followed by 541 nm light (h) irradiated with UV light followed by 517 nm light (magnification 40×).

Most interestingly, it was further observed that after UV irradiation, colors of these two mixed crystals can be tuned carefully to exhibit different shades of orange, red, and pink by changing the irradiation time with each monochromatic light. Fatigue Resistance of Parent and Mixed Fulgides in Crystalline Powder Form. Fatigue resistance of parent fulgides 1E, 2E, 3E and their mixed crystals MIX-1E, MIX2E were studied in the crystalline powder form using solid state UV−visible spectroscopy. A number of coloring and bleaching cycles were performed on each BaSO4 pellet prepared by mixing respective fulgide powders, until the system was mechanically fed up due to the loss of photochromic properties of molecules (Figure S4). With respect to Figure 5, MIX-1E and MIX-2E showed better fatigue resistance with 17 and 16 coloring-bleaching cycles respectively than their respective parent fulgides in the solid state. The number of cycles underwent by 1E, 2E, and 3E were only 14, 12, and 10, respectively, before the systems collapsed completely. Thus, solid state UV−vis spectroscopic studies revealed clearly that upon mixed crystal formation fatigue resistance has been enhanced up to a certain level. Thus, to investigate the probable microscopic cause for this enhancement, molecular structures of three parent fulgides and their mixed crystals were scrutinized. In fulgides, the solid state photochromic electrocyclization reaction is a unimolecular process controlled by mainly topochemical factors.1 Among them, the distance between two carbon atoms involving in photocyclization reaction to form the closed form is the major factor. Generally, fulgides having this distance only in the range of 3.34−3.39 Å are reactive in the solid state1,25 giving colored photoproducts. As depicted in Figure 1, this distance is considerably shorter in both mixed crystals MIX-1E [3.345(4) Å] and MIX-2E [3.352(6) Å] than that distance in their respective parent crystals of 1E [3.373(3) Å] or 2E [3.381(4) Å] or 3E [3.362(7) Å]. Since two bond forming carbon atoms are closer to each other in both mixed crystals than in their pure parental crystals, photocoloring and photobleaching reactions can easily occur with less atomic movements in the mixed ones than in their parent fulgides. This must be one reason to enhance the fatigue resistance in the mixed crystal form. Conversely, such a correlation is not seen between fatigue resistance and distance

Figure 5. Absorbance of the visible peak Vs number of coloring and bleaching cycles for (a) 1E, 2E, and MIX-1E, (b) 1E, 3E, and MIX-2E.

between two reactive carbon atoms in their parent crystals. On the other hand, the average volumes taken up by a single molecule in 1E, 2E, 3E, MIX-1E, and MIX-2E crystal lattices are 343, 330, 341, 333, and 334 Å3 respectively. Thus, upon MIX-1E formation, unit cell volume as well as average volume taken by a fulgide molecule in the mixed crystal lattice has been reduced noticeably with respect to 1E, though 60% the 1E molecules with smaller methyl group (19 Å3) have been replaced by 2E with a larger chloro group (21 Å3).20 Therefore, especially 1E molecules must be more tightly packed in the 3045



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Figure 6. Crystal structures of (a) 2E, (b) MIX-1E, (c) 3E, (d) MIX-2E along the b axis.

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CONCLUSIONS Two photochromic mixed crystals MIX-1E and MIX-2E were successfully formed between two isostructural fulgide pairs by replacing methyl substituted fulgide (1E) molecules with chloro (2E) and bromo (3E) substituted fulgides, respectively. Unit cell parameters of both mixed crystals were closer to those of the fulgide with the methyl group, as it became the host lattice. Formation of mixed crystals leads to multicolor photochromism in the single crystal form imparting four main colors to the crystal. At the same time, because of the results of a shorter moving distance between two reactive carbon atoms forming the bond by electrocyclization and weaker intermolecular interactions, fatigue resistance was enhanced in both mixed crystals compared to their parent fulgides. Therefore, from the available data it can be concluded that such multicoloured photochromic systems with better fatigue resistance can be used for the fabrication of multifrequency 3D optical memory media and photoswitchable full-color displays in the future, if the mixed crystals are made between thermally stable fulgide pairs.

MIX-1E lattice than in its parent crystal lattice, while packing of the 2E molecules in MIX 1E is almost same as in its pure 2E lattice. Similarly, both 1E and 3E molecules must be more tightly arranged in MIX 2E than in their respective parent crystals. This condition can reduce the fatigue resistance in the mixed crystal form, which is contrary to what was observed. However, according to the crystal structures given in Figure 6, both in 2E and 3E, the intermolecular distance between two Cl atoms and two Br atoms of the neighboring fulgides are 3.321 and 3.390 Å respectively, which are much shorter than the sum of the Bondi’s van der Waals radii of two Cl (1.75 Å) and two Br (1.85 Å) atoms, respectively.26 This clearly proves that there are strong Cl···Cl and Br···Br intermolecular interactions in 2E and 3E respectively, but not in 1E. Once 2E and 3E adopt the crystal structure of 1E, only weak H···Cl (2.820 Å) and H···Br (2.697 Å) and C···Br (3.403 Å) interactions are available, respectively, as the respective distances are close to the sum of the van der Waals radii of two respective atoms. Thus, in the mixed crystal form, ring closing and opening reactions of fulgides can occur with less steric hindrance. Therefore, the overall fatigue resistance has been enhanced in mixed crystals as a result of the combined effects arising from the above three factors. Consequently, using such photochromic mixed crystals, not only multicolour photochromism but also enhanced fatigue resistance can be achieved for molecular devices. However, these two mixed systems and their parent fulgides were not thermally very stable due to the presence of a hydrogen atom at the ring closing carbon atom of the benzene ring which can be thermally rearranged to give a colorless product.1 This situation can be restricted by introducing a methyl group to the ortho position of the benzene ring.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01708. HPLC data, UV−vis spectra of parent and mixed crystals before UV irradiation, UV−vis spectra recorded for five crystals to obtain fatigue resistance, PXRD pattern of 1E before UV irradiation and after 15 cycles of UV and white light irradiation (PDF) Accession Codes

CCDC 1518312−1518316 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by email3046



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ing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 94-81-2394439. E-mail: [email protected]. ORCID

Champika V. Hettiarachchi: 0000-0001-8433-7994 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance from National Research Council Grant No. NRC-09-22 and National Institute of Fundamental Studies for providing machine time to record solid state UV- Visible spectroscopic data are kindly acknowledged.

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REFERENCES

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Multicolor Photochromism of Fulgide Mixed Crystals with Enhanced

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